The invention relates to a solar collector in the nature of an arcuate generally parabolic surface which concentrates solar radiation upon an energy absorbing target which is located at the focus of the parabola.
Linear tensioned thin-film, or “membrane” concentrators have many advantages over more traditional designs utilizing rigid backed reflective material. Most importantly they weigh less. The reduced weight allows larger collectors to be mounted on a pair of posts with a specified strength and roofs with a given specified load. In addition, multiple concentrators can be controlled for example rotated, with less energy due to the lower weight. Thus, fewer motors may be required to move them into a desired position.
Linearly tensioned thin-film concentrators are typically supported at each end. The length of the span between supports is one of the most important factors of weight and cost, since the end supports are among the heaviest and most expensive components of the system. Typical systems use a single beam or steel tube to span the length. This approach has three major limitations.
First, the torsional rigidity of a tube varies inversely with the length, i.e. a forty foot tube has ½ the torsional rigidity of a twenty foot tube (assuming other factors are fixed). This makes it very difficult to use a torsion tube to span large distances and thus reduce the cost of a linearly tensioned thin-film concentrator.
Second, the buckling strength of a long torsion tube is generally low and inversely proportional to the square of the length. Since the tube is generally in a horizontal position, it must resist the effect of buckling while it is resisting its own weight as a beam. In addition, linearly tensioned thin-film reflectors place the tube in compression, which significantly increases the buckling forces on the torsion tube. While the buckling strength will increase with fixed ends, it is still of major concern. In order for a single tube to meet the required strength over a long span, the thickness of material and the diameter of the tube used must be increased. The increased weight and cost of a tube to span a long distance often makes it impractical for use in spanning long distances.
Third, using a long torsion tube in the center of a collector pair makes it difficult to attach additional components on the outside edges of the collector pair. One specific limitation this causes is a difficulty to securely attach wind spoilers to deflect wind at the edges of the collectors. Deflecting wind at the outer horizontal edges is critical for collectors that are stowed in the horizontal position.